The present invention relates to a linear motor of the type in which a movable coil moves linearly in a gap formed by opposing permanent magnets.
As driving apparatuses for accurately positioning members within a long stroke of 10-100 cm or so, movable coil-type linear motors as disclosed in Japanese Patent Publication No. 58-49100 and Japanese Utility Model Laid-Open No. 63-93783 are conventionally used in wide applications. In these linear motors, a plurality of permanent magnets magnetized in their thickness directions are arranged in opposite to each other such that their magnetic poles of different polarities face each other, and movable coil assemblies are disposed in gaps formed by the opposing permanent magnets, so that they can move linearly in a direction perpendicular to the magnetic fluxes.
In such linear motors, the magnetic circuits do not have center yokes, and magnetic fluxes constitute a plurality of closed loops in the gaps, avoiding the concentration of the magnetic fluxes in a part of the magnetic path. Accordingly, it is possible to generate a uniform magnetic flux density distribution over the entire length of a long stroke.
FIG. 10 shows a typical conventional linear motor. This linear motor comprises yokes 1, 1 in the shape of a flat plate made of a ferromagnetic material such as a steel plate, a pair of permanent magnet assemblies, and a movable coil 5. Each permanent magnet assembly is constituted by a plurality of permanent magnets 2, 2 . . . magnetized in their thickness directions and arranged longitudinally along the yoke 1, such that N and S magnetic poles appear on their surfaces alternately. The permanent magnets 2, 2 . . . are fixed to each yoke 1, and a pair of yokes 1, 1 are fixed by support members 4, 4, such that the permanent magnets 2, 2 . . . fixed to each yoke 1, 1 are opposing each other via a gap 3 with their opposite magnetic poles facing each other. The support members 4, 4 are preferably made of a ferromagnetic material like the yokes 1, 1. A movable coil 5 is constituted by a flat multi-phase coil having a winding direction perpendicular to magnetic fluxes in the gap 3. Specifically, the movable coil 5 is constituted by a plurality of coils arranged along the longitudinal direction of the permanent magnet assembly while overlapping except for their small parts. The directions of magnetic poles are detected by magnetic flux detectors, etc. to switch coils to which electric current is supplied and to change its direction. Incidentally, the movable coil 5 is integrally supported by a movable member (not shown).
In the above structure, when the movable coil 5 is supplied with electric current, the electric current flowing in the movable coil 5 crosses at right angles the magnetic flux generated by the permanent magnets 2. Accordingly, by Fleming's left hand rule, the movable coil 5 is subjected to a driving force in a longitudinal direction of the yokes 1, 1. Thus, the movable member (not shown) integrally supporting the movable coil 5 moves longitudinally along the yokes 1, 1. Next, when electric current in an opposite direction is supplied to the movable coil 5, a driving force in an opposite direction is generated, thereby moving the movable member in an opposite direction. Accordingly, by controlling electric current supplied to the movable coil 5, the movable member can be moved and stopped at a desired position.
In the linear motor of the above structure, each permanent magnet 2 has a rectangular cross section, so that every permanent magnet 2 has the same cross section in a plane perpendicular to the moving direction of the movable member. In other words, every permanent magnet has the same thickness and the same width. Also, between the adjacent permanent magnets 2, 2, there is a gap or an adhesive layer or short-circuiting of the magnetic flux, resulting in smaller magnetic flux density than that in the center portion of each permanent magnet 2. Accordingly, in the entire length of the permanent magnet assembly, the magnetic flux distribution in the moving direction of the movable coil 5 substantially has a wave form constituted by a repetition of a semi-circle.
In general, in a motor having a rotator constituted by permanent magnets and a stator armature disposed around the rotator, the magnetic flux of the rotator and the magnetomotive force of the armature should be kept perpendicular to each other. For this purpose, it is necessary to obtain an sinusoidal armature current and a sinusoidal magnetic flux distribution in the gap between the permanent magnet rotator and the stator armature by a proper controlling circuit. By meeting this requirement, a torque generated by the motor depends only on the product of maximum values of the armature current and the magnetic flux density regardless of the displacement angle of the rotator from a reference axis. In other words, a torque ripple due to the above displacement angle can be prevented.
On the other hand, the structure of a linear motor is considered that the above rotator and armature have infinite diameters. Accordingly, the above principle is applicable. Thus, if the magnetic flux density distribution of the permanent magnet assembly in the moving direction of the movable coil 5 in FIG. 10 has a sinusoidal wave form, the torque ripple due to the movement of the movable coil 5 can be eliminated, leading to a linear motor having excellent linearity.
However, in the above conventional linear motor, the magnetic flux density distribution in the moving direction of the coil 5 is not in a sinusoidal wave form due to the shapes of permanent magnets 2, resulting in torque ripple and poor linearity, which in turn leads to low positioning accuracy.
Japanese Utility Model Laid-Open No. 63-93783 discloses a dc linear motor in which permanent magnets opposing an armature have smooth recesses at boundaries of adjacent magnets having opposite magnetic poles to reduce a ripple of a driving force.
However, since this linear motor has a position detector (magnetoelectric conversion element) for each coil to reverse the direction of current supplied to the coil, each coil should have a current control means. Accordingly, when it has a large number of coils to obtain a larger driving force, its structure becomes rather complicated.
In addition, although there are smooth recesses (almost circular in cross section) at boundaries between adjacent magnets, magnetic flux is likely to be concentrated in such recesses, resulting in a reduced peak value of magnetic flux density. Also, the resulting magnetic flux density distribution has a relatively sharp wave form. This reference further mentions that in this linear motor, sinusoidal current is supplied to the coils. However, this sinusoidal current is obtained by the influence of the sinusoidal magnetic flux distribution on the outputs of the position detectors, and this does not change the fact that this linear motor is a dc-linear control type.